This is what ECG paper looks like. This paper is made of several small blocks measuring 1 mm x 1 mm. These blocks are organized into bigger blocks, each measuring 5 mm x 5 mm.

Voltage is measured along the vertical axis of the paper. In the standard setting, one small block measures 0.1 mV, meaning a big block measures 0.5 mV. However, this setting can be adjusted on ECG machines to change the resolution of the voltage axis as deemed necessary.

Time is measures along the horizontal axis of the paper. The standard ECG paper speed is 25 mm/s, meaning that one small block measures 40 ms, one big block measures 200 ms, and the entire length of the paper measures 10 seconds worth of data. Once again, this setting can be adjusted on ECG machines to change the resolution of the time axis. 

It is crucial to always check the resolution of the voltage and time axes to avoid inaccurate interpretation. For example, a paper speed of 50 mm/s makes everything appear twice as wide.

ECGs are traced from left-to-right. ECG graphs running parallel to one another are recorded simultaneously, as shown above.

The fact that parallel rhythm strips on an ECG are traced simultaneously enables you to map between parallel rhythm strips on an ECG just by moving vertically. This is a very powerful technique in ECG interpretation.

For example, it may be difficult to tell what's what in the following strip: